I have had that happen a couple of times. Both times a piece of the vinyl dowel inside the awning has been broken off. I replaced it with a similar sized wood dowel.
1. With awning open about a foot, relieve the pressure (easiest way is to have wife or someone else push in on awning roller.
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2. If vinyl dowel end is missing replace. then push the awning retaining section back into the slot (I have used a screwdriver to expand the opening).
3. Once the awning retaining tube is in place, us vice grips or a small C clamp to compress the retaining slot so it is almost closed to keep the retaining tube from escaping.
Usually takes me longer to round up the tools, ladder, dowel, etc. than it does to fix it.